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coherent tunable radiation in quadratic media. In large-area devices, the parametrically interacting fields experience diffraction which yields to spon-taneous pattern formation in their transverse sec-including low-noise measurements and detection [2] or optical coherent information processing [3]. In degenerate optical parametric oscillators (OPOs) the energy conversion process is frequency degenerate and the emerging pattern can be, at the primary instability, either homogeneous or spa